The invention relates in general to analog-to-digital (A/D) converters and in particular to photonic analog-to-digital (A/D) converters.
The majority of the signals encountered in nature are analog while the preferred method of processing these signals is digital. Digital signal processors provide higher resolution, improved flexibility and functionality, and improved noise immunity over their analog counterparts. As a result, the A/D interface is generally considered to be the most critical part of, any signal acquisition and processing system. Because of the difficulty in achieving high-resolution and high-speed A/D converters, the A/D interface has been and continues to be a barrier to the realization of high-speed, high-throughput systems.
Electronic A/D techniques have been investigated but appear to be limited. xcexa3xcex94 techniques have been successful in providing high-resolution converters but only for audio frequency signals. Optical approaches have also been investigated to leverage the wide bandwidth and parallelism of optics but most have been limited by the component linearity, device speed, or dynamic range
Over the past seven years, performance of electronic analog-to-digital (A/D) converters has improved by only one-bit for high-performance applications (See R. H. Walden, xe2x80x9cAnalog-to-digital converter technology comparison,xe2x80x9d Proc. GaAs IC Symposium, 217-219 (1994)). As a result, there has recently been a renewed interest in new and innovative approaches to A/D conversion, with a significant emphasis on photonic techniques. The potential advantages of using photonics technology come in the form of high-speed clocking, broadband sampling, reduced mutual interference of signals, and compatibility with existing photonic-based systems. Another advantage of processing signals in the optical domain is the parallelism obtained by performing signal processing in both space and time domains simultaneously. Photonic approaches to A/D conversion have been considered in the past with varying degrees of success. Some of the approaches employed Mach-Zehnder interferometers, others incorporated acoustooptic modulators, and recently multiple quantum well modulators have been incorporated into non-traditional architectures.
Probably the best known approach to A/D conversion using photonic techniques was developed by Taylor in 1975 (See H. F. Taylor, xe2x80x9cAn electrooptic analog-to-digital converter,xe2x80x9d Proc. IEEE 63, 1524-1525 (1975). He was the first to recognize that the periodicity of the output of an interferometric electrooptic modulator with applied field was homomorphic to the periodic variation of a binary representation of the applied field. A 4-bit implementation (See R. A. Becker, C. E. Woodward, F. J. Leonberger, and R. W. Williamson, xe2x80x9cWide-band electrooptic guided-wave analog-to-digital converter,xe2x80x9d Proc. IEEE 72, 802-819 (1984)) of this approach is shown in FIG. 1(a).
The basic optical component used in this architecture is a channel waveguide version of a Mach-Zehnder interferometric modulator. It is easily shown that the output intensity of a single interferometer varies as                     I        =                                            I              ⁡                              (                0                )                                      ⁢                                          cos                2                            ⁡                              (                                                      φ                    2                                    +                                      ϕ                    2                                                  )                                      ⁢                          xe2x80x83                        ⁢            with            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            φ                    =                                    2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              (                                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    n                                    λ                                )                                      =                          k              ⁢                              xe2x80x83                            ⁢              L              ⁢                              xe2x80x83                            ⁢              V                                                          (        1        )            
where (xcfx86/2 is the static phase difference between the two paths and xcfx86/2 is the electrooptic phase difference. Here, xcex94n is the chance in refractive index, V is the applied voltage, L is the modulator length, and k is a constant which depends on the electrooptic parameters of the crystal, the electrode spacing, and optical wavelength.
In FIG. 1(a), the analog input signal V is applied in parallel to one arm of each of the four modulators, one for each bit of resolution. The optical output from each modulator is then detected by an avalanche photodiode. The signal from each modulator is then compared to a reference signal, obtained from the common light source. The output of each comparator is either a binary 1 or 0, depending on whether the modulator output intensity is greater than or less than 1(0)/2, respectively. The output of the top modulator represents the least significant bit (LSB) in the digital word and that of the bottom modulator is the most significant bit (MSB). The output intensity threshold, and corresponding binary representation for each modulator are shown in FIG. 1(b).
This electrooptic A/D converter provides several distinct advantages including linear complexity and decoupling of the analog sampled signal from the optical sampling signal. A fundamental limitation is that each additional bit of resolution requires a doubling of the electrode length of the least significant bit modulator. In LiNbO3, this produces a transit-time limitation on performance of approximately 6 bits at 1 GHz.
Other types of optical A/D converters have also been investigated, but with much less success than Taylor""s electrooptic converter. Most were limited by speed, complexity, or resolution and therefore did not warrant further investigation. For example, Tsunoda proposed optical A/D conversion based on a matrix-multiplication formalism (See Y. Tsunoda and J. W. Goodman. xe2x80x9cCombined optical A/D conversion and page composition for holographic memory applications,xe2x80x9d Appl. Optics 16, 2607-2609 (1977)). In this implementation, an astigmatic optical processor (See L. J. Cutrona, Optics and Electro-Optical Information Processing, ch. 6, Cambridge, Mass.: MIT Press (1965)) was used with the electronic analog input signal driving an optical beam deflector. This method of optical A/D conversion was limited by the speed capacity of the deflector, C=K/xcfx84, where K is the number of resolvable spots addressable and xcfx84 is the time required for random access to a specific location. Other approaches to the optical A/D conversion problem can also be found in the literature.
The present invention provides a method of converting an analog signal to a digital signal comprising (a) filtering the analog signal to the range 0xe2x89xa6fxxe2x89xa6fB; (b) sampling the filtered signal at a rate fB greater than  greater than fN, where fS is the sampling frequency, fN=2fx is the Nyquist frequency of the sampled signal, and fBxe2x89xa6fS/2 is the constrained signal bandwidth; (c) converting the sampled signal to an optical sampled signal; (d) converting the optical sampled signal from a temporal signal to a spatial signal; (e) illuminating a smart pixel array with the spatial signal; (f) processing the spatial signal with an error diffusion neural network to produce a 2-D binary image; and (g) averaging rows and columns of the 2-D binary image using a digital low pass filter and a decimation circuit.
The present invention also provides an A/D converter comprising a filter for filtering an analog signal to the range 0xe2x89xa6fxxe2x89xa6fB: a sampler for sampling the filtered signal at a rate fS greater than  greater than fN, where fS is the sampling frequency, fN=2fx is the Nyquist frequency of the sampled signal, and fBxe2x89xa6fS/2 is the constrained signal bandwidth, the sampler also converting the sampled signal to an optical sampled signal; a temporal-to-spatial converter for converting the optical sampled signal from a temporal signal to a spatial signal; a smart pixel array illuminated by the spatial signal; an error diffusion neural network for processing the spatial signal to produce a 2-D binary, image; a digital low pass filter; and a decimation circuit.
Another embodiment of the invention is a method of converting an analog signal to a digital signal comprising (a) filtering the analog signal to the range 0xe2x89xa6fxxe2x89xa6fB; (b) sampling the filtered signal at a rate fS greater than  greater than fN, where fS is the sampling frequency, fN=2fx is the Nyquist frequency of the sampled signal, and fBxe2x89xa6fS/2 is the constrained signal bandwidth; (c) converting the sampled signal to an optical sampled signal; (d) converting the optical sampled signal from a temporal signal to a spatial signal; (e) illuminating a smart pixel array with the spatial signal: (f) processing the spatial signal with an error diffusion neural network to produce a series of 2-D binary images: and (g) averaging the series of 2-D binary images in x, y and z directions using a digital low pass filter and a decimation circuit.
The invention also encompasses an A/D converter comprising a filter for filtering an analog signal to the range 0xe2x89xa6fxxe2x89xa6fB; a sampler for sampling the filtered signal at a rate fS greater than  greater than fN, where fS is the sampling frequency, FN=2fx is the Nyquist frequency of the sampled signal, and fBxe2x89xa6fS/2 is the constrained signal bandwidth, the sampler also converting the sampled signal to an optical sampled signal; a temporal-to-spatial converter for converting the optical sampled signal from a temporal signal to a spatial signal; a smart pixel array illuminated by the spatial signal; an error diffusion neural network for processing the spatial signal to produce a series of 2-D binary images; a digital low pass filter; and a decimation circuit.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the following drawing.